It was a crumb of an idea that he enjoyed for nostalgic reasons, fondly remembering his time as a kid hanging out in front of liquor stores trying to get adults to buy booze for him and his friends, but where his interest really took hold was in exploring Sue Ann as a real character and giving her not only real motivation, but an in-depth backstory.
And in that sense, the script needed a lot of work.
That simple hook was apparently enough to draw Octavia Spencer to Ma, but Tate Taylor further stressed that what he had in mind for the project was a lot more than just having her as a psychotic serial killer who murders teens for essentially sport.